Information is transmitted in an optical channel using optical modulation. In a receiver, the received optical signal contains the information, which could have been modulated in amplitude, phase, polarization or a combination thereof. The demodulation of the optical signal is done using a transducer, e.g. photodiode, that converts from the optical to the electrical domain. The transducer delivers an electrical signal, which is processed to extract the information contained in the optical signal. The maximum to minimum optical power, i.e. dynamic range, together with the transducer define the received electrical signal dynamic range. Modern communication systems use complex modulation schemes, e.g. quadrature modulation, to increase the communication channel efficiency. The efficiency of complex modulation schemes is proportional to the maximum operation frequency, i.e. baud rate, and dynamic range.
The received optical signal is transformed to an electrical signal using a transducer. Most transducers convert optical signals to electrical current. However, the current magnitude, which is proportional to the received optical power, needs to be amplified. Typically, during amplification the electrical current is converted to a voltage. The current-to-voltage amplifier, or transimpedance amplifier, remains one of the most challenging components in an optical receiver.
Large values of optical power, define high magnitude currents at the transducer output. High current at the TIA input will create distortion at the TIA output, typically due to non-linear behavior of the components used for amplification. The maximum input current a TIA can amplify, with a distortion below a defined maximum, defines the upper boundary of the TIA dynamic range.
The distortion generated in broadband signals can be quantified, by adding all the harmonics generated when a single tone signal is used at the input. The ratio of the output signal power, at the input signal tone frequency, to the total added power of the harmonics in the output signal is the total harmonic distortion, or THD.
On the other hand, small values of optical power, define small magnitude currents at the transducer output. However, small currents at the TIA input will be more sensitive to noise added by the TIA during amplification. The lower boundary of the TIA dynamic range is defined by the minimum input current that can be amplified, while still enabling the information to be recovered, even with the added TIA noise.
A variable gain TIA requires gain control, whereby the receiver can modify its characteristics to process high power incoming signals with a defined maximum distortion, and low power incoming signals with a defined maximum noise. Extending the TIA dynamic range is a key to increasing the receiver's dynamic range, and therefore increase the channel capacity.
Variable gain TIAs implement variable gain in their designs. Typically, the maximum gain of the TIA is not sufficient amplification, therefore, variable voltage-to-voltage amplifiers (VGAs) are cascaded in the receiver. VGAs contribute to limit the upper boundary of the dynamic range because they add distortion when large signals are amplified, generally at low gain values. Moreover, they also contribute to limit the lower boundary of the dynamic range because they add noise when small signals are amplified, generally at high gain values.
Individual optimization of each TIA and VGA is based on the behavior of each block at a given gain setting. Optimization of the overall chain, including the TIA and one or more VGAs, considers the assignment of gain, and bandwidth per block for a targeted receiver response. A variable gain TIA limits the receiver's dynamic range; therefore, increasing the maximum input for a given maximum added distortion at the output, becomes essential to extend the receiver's dynamic range.
An object of the present invention is to overcome the shortcomings of the prior art by providing a receiver including a TIA with variable gain for maximum input signal dynamic range, whereby the gain may be reduced for large input current signals, while adding the minimum amount of distortion, and whereby the gain may be increased for small input current signals, while adding the minimum amount of noise.